The development of the vertebral column is an ideal model to study positional specification mechanisms that create differences in embryonic cell populations. Dr. Rosenthal's laboratory has recently shown that the axial specification of myogenic lineages in developing mouse somites is reflected in the expression of a muscle specific transgene. The initial positional differences which appear as a rostrocaudal gradient are retained during development, providing a molecular marker for the generation and persistence of positional cues. The molecular basis of this gradient is not yet known. I propose to identify spatially restricted muscle gene regulators by implementing an unbiased search for these molecules. In order to obtain sufficient material for identifying and characterizing positionally restricted regulators in the myotome, I will generate a transgenic mouse expressing a vital marker, the green fluorescent protein, exclusively in the myotomal compartment under the control of the Myogenin promoter. Expression can be directly observed in the developing embryo or in cells purified by means of a fluorescence activated cell sorter. This experimental design will allow me to 1)dissect different axial subsections of the vertebral column; 2)isolate pure populations of myotomal cells; 3)identify differentially expressed RNAs; and 4)analyze the corresponding genes for known as well as novel factors potentially involved in the regulation of positional cues.